Effect of contrail overlap on radiative impact attributable to aviation contrails

Abstract. Condensation trails (“contrails”) which form behind aircraft are estimated to cause on the order of 50 % of the total climate impact of aviation, matching the total impact of all accumulated aviation-attributable CO2. The climate impacts of these contrails are highly uncertain, in part due to the poorly-understood effect of overlap between contrails and other cloud layers. With the airline industry projected to grow by approximately 4.5 % each year over the next 20 years, instances of contrail overlap are expected to increase, including any potential mitigating or amplifying effects on contrail-attributable radiative forcing. However, the impacts of cloud-contrail overlaps are not well understood, and the effect of contrail-contrail overlap has never been quantified. In this study we develop and apply a new model of contrail radiative forcing which explicitly accounts for overlap between cloud layers. Cloud-contrail overlap is found to be responsible for 93 % of net radiative forcing attributable to 2015 contrails. We also find significant variation in the sensitivity of contrail radiative forcing to cloud cover with respect to geographic location. Clouds significantly increase warming at high latitudes and over sea, transforming cooling contrails into warming ones in the North-Atlantic corridor. Based on the same data, our results indicate that disregarding overlap between a given pair of contrail layers can result in longwave and shortwave radiative forcing being overestimated by up to 16 % and 25 % respectively, with the highest bias observed at high optical depths (> 0.4) and high solar zenith angles (> 75°). When applied to estimated global contrail coverage data for 2015, contrail-contrail overlap reduces both the longwave and shortwave forcing by ~ 2 % relative to calculations which ignore overlap. The effect is greater for longwave radiation, resulting in a 3 % net reduction in the estimated RF when overlap is correctly accounted for. This suggests that contrail-contrail overlap effects can likely be neglected in estimates of the current-day environmental impacts of aviation. However, the effect of contrail-contrail overlap is likely to increase in the future as the airline industry extends into new regions, intensifies in existing regions, and invests in higher-efficiency engines which are thought to promote contrail formation.